Mechanically driven motor vehicle coolant pumps are mechanically coupled with the internal combustion engine and are therefore not driven as required. In contrast thereto, an electrically driven coolant pump for a motor vehicle can be driven exactly so as to meet demands and, in particular, to convey coolant while the motor vehicle is in operation but while the internal combustion engine is temporarily deactivated. In order to avoid shaft seals that may always be the source of leakages, the drive motors used for electric coolant pumps are so-called gap-tube drive motors in which a cylindrical gap tube separates the dry motor stator formed by the motor coils in a fluid-tight manner from the wet motor rotor arranged in the cooling medium. Gap tubes of plastic material are used in practice almost exclusively because they can be manufactured at low cost and have neutral magnetic properties.
It has, however, been found that in practice diffusion of water or water vapor through a plastics gap tube cannot be entirely avoided unless substantial efforts are taken. By taking suitable measures, for example, encapsulation, it is possible to sufficiently protect the motor coils against humidity diffusing through the gap tube, but, depending on the respective structure, humidity cannot be prevented from reaching the motor electronics consisting of the commutation elements and power semiconductors.
DE 20 2011 051 526 U1, DE 10 2010 011 316 A1 and DE 200 07 099 U1 therefore describe using non-magnetic sheet metal or high-grade steel for the gap tube. These general suggestions were not, however, carried out in practice since demanding a corrosion-resistant high-grade steel that also has advantageous magnetic and electric properties seemed a paradox. Such a solution could also not be realized at low cost.